Optical systems are widespread and often include a light source or sources and a system of optical elements. In some optical systems, a light beam originating from a light source or an assembly of light sources may be split into two or more beams traveling in different directions, which may or may not be recombined further downstream. In other types of optical systems, light beams originating from different light sources or light source assemblies may initially travel in different directions, and also may or may not be recombined further downstream. Such multi-directional optical systems are usually relatively complex and include large numbers of various optical elements, such as refractive, reflective and diffractive optical elements. Folding mirrors are commonly used to fold the optical paths and thus reduce the footprints of large complicated optical systems.